The present invention relates to a multi-layer image mixing apparatus for creating a composite image from multiple image layers.
An image mixing technique is disclosed in U.S. Pat. No. 4,951,229. According to this technique, a process of selecting one of a plurality of pixels, which have been obtained from multiple bit planes in parallel, is repeatedly performed, thereby creating a single combined image. These pixels are selected with reference to the display priorities assigned to respective bit planes.
This prior art is advantageous in that a composite image can be created using no buffer memories. However, since just one pixel is selected from a number of pixels, the technique is not applicable to semitransparent mixing of multiple layers.
It is therefore an object of the present invention to implement semitransparent mixing of multiple layers by using a small-sized image memory for image mixing.
To achieve this object, according to the present invention, multiple layers are processed sequentially using the small-sized image memory repeatedly. The semitransparent mixing is implementable by weighting a pixel value associated with one or more processed layers and stored in the image memory and a pixel value associated with a next layer and adding these weighted values together.
Specifically, the present invention provides a multi-layer image mixing apparatus for creating a composite image from multiple image layers. The apparatus includes: an image memory; and input means for sequentially inputting image fractions of the multiple image layers from foremost through rearmost ones. Each of the image fractions is located at the same position in associated one of the layers and has a size of one frame or less. The apparatus further includes: initializing means for initializing the image memory by storing the image fraction of the rearmost layer in the image memory; and mixing means for performing the process steps of a) mixing one of the image fractions that was stored previously in the image memory with another one of the image fractions that has just been input by calculating a weighted average of these two image fractions and b) storing the newly mixed image fraction in the image memory. The mixing means repeatedly performs the process steps a) and b) until the mixing means has processed the image fraction of the foremost layer. The apparatus further includes: output means for outputting a combined image fraction that has been finally stored in the image memory by the mixing means; and control means for making the input, initializing, mixing and output means perform their processes continuously until the last image fraction of the foremost layer has been processed.
The image memory for use in image mixing may have a storage capacity equivalent to one line of a raster-scan display device, for example. In such a case, the image fraction has a size of one line, and a frame, which is made up of the combined lines that have been sequentially output from the image memory, is presented on the display device. It should be noted that the storage capacity of the image memory can be determined irrespective of the number of layers to be combined.